Creation of microstructured spectacle lenses in prescription lens production

ABSTRACT

A method for providing spectacle lenses that have a microstructure on at least one side. A procedure is described, in which the structure is introduced into a face of the lens after the processing of the face of the lens. Processing steps used to achieve the optical quality of the corresponding surface can be omitted. The back face of a spectacle lens may be microstructured by CNC-controlled turning after a first shaping processing, wherein the polishing step is omitted.

The present invention relates to a method for producing a lens, in particular a spectacle lens, that has a microstructure, in particular a diffractive microstructure, on at least one lens surface.

Diffractive optics often use a combination of a refractive effect and a diffractive effect on at least one surface or boundary surface. This allows very good imaging qualities to be achieved even in complex optics with a high functional density just with comparatively few lens elements. Especially for applications in which only a single lens is used, imaging errors that can otherwise only be suppressed by multilayered lenses or by lenses with refractive index gradients can be reduced by a combination of a refractive effect and a diffractive effect. In particular in the case of lenses, such as spectacle lenses for example, that are constructed from a dispersive material, occasionally diffractive microstructures are used on a lens surface, in order for example to reduce color fringes caused by chromatic aberrations of the lens. Some examples of a design of diffractive structures for color fringe correction are known for example from DE 10 2010 051 627 A1, DE 10 2010 051 637 A1, DE 10 2010 051 645 A1 and DE 10 2010 051 762 A1.

However, one of the challenges when using diffractive structures is the high degree of complexity involved in their production. In particular for the production of rotationally symmetrical structures, machining methods (for example diamond turning) have become established. In these methods, either the lens to be produced may be machined directly, or a casting mold or a stamp is machined, in order to form in it a negative of the microstructure that is to be provided on the lens, which is then transferred onto the corresponding lens surface by molding of the lens or by stamping or embossing.

On account of their size (typically 0.3-5 μm axially, 1-500 μm laterally), such microstructures are often very sensitive and are easily impaired in their optical effect by contaminants and/or instances of damage (scratches). Especially when a lens is provided with a microstructure on one lens surface, while the other lens surface still has to undergo further machining operations, such as milling, turning, grinding and/or polishing, the microstructure already produced, for example by molding of the lens blank, has to be protected from damage and contaminants.

Particularly important and widespread is the machining of for example molded lens blanks in the production of spectacle lenses from semifinished products, the machining being used for example to create an individual prescription lens surface (also referred to as a prescription surface). Important steps of this prescription lens production (PLP) are described by way of example below.

Thus, for example, first a semifinished product is molded. On this, one surface (typically the front surface), which is also referred to hereinafter as the primary surface, already has the final geometry, in particular with regard to the refractive properties such as the distribution of spherical and cylindrical effect(s), and possibly even the final surface finish (for example with regard to the desired low roughness, possibly even including a microstructure).

The geometry here may typically be spherical (for example generally in the case of single vision lenses and in the case of back-surface progressive lenses), toric or atoric (in the case of single vision lenses or special back-surface progressive lenses to improve the imaging properties), already include a cylindrical component or already have a progressive profile (in the case of front-surface progressive lenses or lenses with progressive components on both surfaces).

In a further step, the surface facing away from the primary surface (i.e. typically the back surface), which is also referred to hereinafter as the prescription lens surface (PLP surface), is provided with the desired surface geometry. This may be produced by classic tools (for example concave truing tools) or with the aid of (computer-aided) free forming machines.

When using classic tools for classic grinding, typically sphero-cylindrical combinations are produced with the aid of special concave truing tools. Correspondingly, this technology is typically used for simple single vision lenses, single vision lenses with specially optimized primary surfaces and progressive lenses with a progression on the front surface. The process generally consists of three steps. The first step is carried out by means of milling and involves removing a comparatively great amount of material (for example locally up to about 15 mm). In the second step, the surface is finely ground by means of a concave truing tool. Correspondingly, after this step the workpieces have a great roughness (for example RZ about 8-12 μm, RA about 0.3-0.6 μm). Correspondingly, this is followed by a further process, polishing, in which, with little removal (for example about 10-20 μm), the surface is provided with the desired form with the corresponding roughness (RZ about 1-2 μm, RA about 0.08-0.25 μm).

The classic free forming technique (milling-grinding-polishing; unimetrics/polymetrics) is a free forming technology with which virtually any desired surfaces can be produced. Correspondingly, this technology is used in particular in the case of progressive lenses in which at least part of the progression and the sphero-cylindrical effect are on the PLP surface. Furthermore, with this technology effect-optimized lenses (for example Multigressiv), individually optimized lenses and lenses with an optimized sphero-cylindrical effect as well as continuous vision lenses and single vision lenses can be produced. The primary surface is in this case usually spherical, but may also include at least part of the progression.

The process of the classic free forming technique generally consists of the three steps—milling or rough grinding, fine grinding and polishing, for which tools with different roughnesses and speeds are used. Consequently, varying amounts of removal and surface qualities (achievement of the form and roughness) are obtained:

-   -   milling or rough grinding         -   (removal: up to about 15 mm; roughness RZ>20 μm; RA>5 μm)     -   fine grinding         -   (removal: up to 0.2 mm; roughness RZ: 4-10 μm; RA 0.5-1 μm)     -   polishing/finishing         -   (removal: 4-20 μm; roughness RZ: 1-2 μm; RA 0.05-0.4 μm)

For an alternative process, known as the cut-to-polish process (cutting-polishing), what has been said above about the classic free forming technique applies in principle:

-   -   milling         -   (removal: up to 15 mm; roughness RZ>20 μm; RA 5 μm)     -   turning         -   (removal: max. 0.2 mm; roughness RZ: 4-10 μm; RA 0.2-0.6 μm)     -   polishing         -   (removal: 4-10 μm; roughness RZ: 0.5-2 μm; RA 0.03-0.2 μm)

Irrespective of the machining technology used, the blank (that is to say the unfinished or semifinished product) is usually blocked. Low melting metal alloys (known as alloys) in combination with adhesive films or special coating layers and a substrate may be used for example for this. After that, the surfaces or the lens may be improved (for example tinting, applying hard, AR or topcoat layers).

The object of the present invention is to simplify the production of individually produced lenses, in particular spectacle lenses, with a diffractive microstructure. This object is achieved by a method with the features that are specified in claim 1. Preferred embodiments are the subject of the dependent claims.

Thus, the invention offers a method for producing a lens, in particular a spectacle lens, where initially a lens blank is provided, in particular in the form of a semifinished product. This may take place for example by molding the lens blank. Polymerizable material that cures after the molding, for example by polymerization, may be used in particular as the material of the lens blank. The lens blank is preferably produced (for example molded) as a semifinished product in which one lens surface (it is also referred to here as the primary surface) already has the ultimately desired form, without for example any formative machining of this lens surface still being required or performed after the molding. In particular for the production of a spectacle lens, the front surface of the spectacle lens is preferably intended as the primary surface.

The surface opposite from the primary surface (also referred to here as the secondary surface) however still has to be adapted with regard to its refractive properties to the special surface specifications that are specified by means of a prescription, which establishes refractive properties of the lens or the secondary surface. The lens blank or the semifinished product is therefore not yet adapted completely to the required refractive properties with respect to the secondary surface. One reason for this is that under some circumstances it would be a very complex matter to mold an already completely adapted lens for every required optical effect, such as for example for individually produced spectacle lenses. For this, the casting mold would already have to be adapted to the optical effect that is required in the individual case. This may however be very complex and expensive. Therefore, even conventionally it is already usual only subsequently to adapt at least one surface in accordance with the specifications of the prescription by machining. The surface then corresponding to the optical effects required by the specifications of the prescription is also referred to as the prescription surface or prescription lens surface. As already stated above, such prescription lens surfaces are conventionally produced or machined by PLP processes.

By contrast, the present invention now proposes machining at least one lens surface of the lens blank to create a prescription lens surface of the lens, the machining of the at least one lens surface comprising creation of a diffractive microstructure. The microstructure serves in particular as a diffractive grating for visible light. As already described above, diffractive microstructures are certainly very advantageous for example for the correction of undesired chromatic aberrations of a lens (for example color fringe correction). In the context of the present invention, it has indeed been recognized that the implementation of the creation of such diffractive microstructures in the production of the prescription lens surface is accompanied by significant technical production-related advantages. Consequently, all that is necessary to both the advantages of an individually adapted refractive effect provided by the prescription lens surface and also the advantages of the diffractive effect provided by the microstructure is to machine a single surface. It has been found here that it is possible in particular to dispense with very time-consuming polishing of the prescription surface, since the microstructure may either already be formed in a single working method involving corresponding material removal for the formation of the prescription surface on the lens blank or be formed after the material removal in a single working step on a still comparatively rough surface thereby created, and in this way offers the required surface quality. For this purpose, the diffractive microstructure is formed for example by material removal or forming, in particular directly in the material of the lens blank.

The method according to the invention is particularly advantageous in the production of spectacle lenses that are to be produced individually. In this case, the back surface (surface on the eye side) of the spectacle lens is preferably machined as the prescription lens surface. A spherical surface (for example generally in the case of single vision lenses and in the case of back surface progressive lenses) or a toric or atoric surface (in particular a single, rotationally symmetrical surface) (for example in the case of single vision lenses or special back surface progressive lenses to improve the imaging properties) or a surface that already has a cylindrical component and/or already has a progression profile (in the case of front surface progressive lenses or lenses with progressive components on both surfaces) is preferably used as the front surface.

The prescription surface is created from a surface of the lens blank by subsequent machining. For this purpose, the corresponding surface of the lens blank is modified to such an extent that it has the required refraction properties. For spectacle lenses that are to be produced individually, individual calculations of the prescription surface are usually carried out before the machining of the prescription surface. Such calculations are based for example on iterative wavefront- and/or ray-tracing computations or calculations by means of local wavefronts. From the results of such methods of calculation or optimization, the required geometry of the prescription surface for achieving the required refraction can be derived, for example in the form of sagittae, the diffractive microstructures not yet necessarily having to be calculated at the same time here.

However, the prescription surface thus calculated may well generally still deviate from the corresponding surface of the lens blank, so that removal of a significant amount of material, corresponding to a material thickness that lies at least above the structure size of the diffractive microstructure, is still required, at least at some locations of the surface. The required material removal may locally even be many times greater than the structure size of the microstructure (in the direction of removal). In order to achieve the required refractive properties, therefore, more material is removed, in particular locally (that is to say at some locations), for the forming of the prescription surface than would be necessary for just forming the microstructure. Consequently, the machining of the at least one lens surface for the creation of the prescription surface particularly comprises removal of material with a thickness of at least partially (that is to say at least locally) at least about 20 μm, preferably at least about 0.1 mm, still more preferably at least about 0.5 mm, most preferably at least about 2 mm.

As still to be explained below, the removal of this material may in this case take place for example separately from the forming of the diffractive microstructure, in a working step of its own, or together with the forming of the diffractive microstructure, in a common working step, in particular by means of the same tool.

Thus, in a preferred embodiment, the machining of the at least one lens surface for the creation of a prescription lens surface comprises milling and/or turning and/or grinding. Preferably, the creation of the microstructure in this case takes place after the milling or turning or grinding, without any interim polishing of the at least one lens surface. In particular, preferably no polishing of the prescription surface takes place after the milling or turning or grinding. Rather, the invention has recognized that adequate surface quality is already brought about by the forming of the diffractive microstructure on the basis of the prescription surface that is created after the milling or turning or grinding. In this case, the roughness of the prescription surface (directly) before the creation of the microstructure may have at least partially (that is to say locally) an R_(z) value of at least about 5 μm, preferably at least about 10 μm, still more preferably at least about 20 μm and/or an R_(a) value of at least about 0.5 μm, preferably at least about 1 μm, still more preferably at least about 2 μm, most preferably at least about 5 μm. Tactile and in particular optical methods (for example a laser scanning microscope) come into consideration in particular for measuring the roughness. In this case, in one aspect the R_(z) value (or the RZ value) may represent the mean surface roughness and the R_(a) value (or the RA value) may represent the average roughness.

For the milling or turning or grinding, preferably the corresponding conventional process steps that have already been mentioned at the beginning may be used, though dispensing in particular with corresponding polishing. Particularly preferably, in particular depending on the required material removal, only one of the methods mentioned (milling or turning or grinding) is used before the microstructure is formed. Thus, the surface quality (roughness) created during milling is sufficient to obtain an adequate optical quality of the lens to be produced by creating the microstructure on the basis of this surface quality.

The creation of the microstructure then preferably takes place by machining or forming. In the case of machining, CNC milling or CNC turning may be used. In the case of turning, a technology of turning that is ultrasound assisted in the axial direction or in the cutting direction is preferably used (see for example S. Hannig, C. Brecher and C. Wenzel: “Schnelle Auslegungsmethode für ultra-schallunterstützte Zerspanung [rapid design method for ultrasound assisted machining]”, MaschinenMarkt (Sep. 13, 2012) http://www.maschinenmarkt.vogel.de/themenkanaele/produktion/zerspanungstechnik/articles/378155/ (as of Feb. 11, 2013); or C. Brecher, F. Klocke, M. Winterschladen, M. Heselhaus: “Ultraschallunterstütztes Hartdrehen für die Fertigung von gehärteten Präzisionsstahlbauteilen [ultrasound assisted hard turning for the production of hardened precision steel components]”, Werkstattstechnik online, year 96, issue 6, pages 396 ff, VDI-Verlag (2006)). Diamond is preferred as the material for the tool. The tool itself may be fashioned in the form of a radius, preferably in the form of a half radius. Typical tool radii in this case range from about 50 μm to preferably about 3 μm, depending on the structure size. In order to improve the removal or reduce tool wear, two- or multi-stage procedures may also be used, with decreasing tool radii and increasing surface quality.

Pressing, possibly with thermal assistance, is preferably used as a forming method.

In particular, the creation of the diffractive microstructure may take place by machining with material removal that is at least partially (locally) greater than the structure size of the diffractive microstructure in the direction of removal (axially). Thus, the creation of the microstructure may comprise removal of material with a thickness of at least partially (that is to say at least locally) at least about 20 μm, preferably at least about 0.1 mm, still more preferably at least about 0.5 mm, most preferably at least about 2 mm. In particular in these cases, depending on the required material removal for creating the prescription surface from the lens blank, the required removal of material may even take place in a single step together with the forming of the microstructure. Therefore, under some circumstances it is no longer required in this case to perform a preceding milling, turning or grinding step.

A significant economic advantage of the invention is that, on the basis of the material removal (or material redistribution) involved in the machining to produce the microstructure, at least the last step of the PLP machining can be omitted. In comparison with the aforementioned conventional methods, this in each case concerns at least the polishing step; in comparison with the classic free forming technique, possibly even the grinding can be omitted. This is advantageous in particular when using free forming technology, since surface-true grinding or polishing is conceptually complex here.

Given sufficiently preformed blanks, when cutting the structure with sufficiently great material removal or when forming with correspondingly great material redistribution, even all of the preceding PLP steps can be omitted and the final surface can be produced directly by the creation of the microstructure (in particular by the technologies mentioned).

The method preferably comprises, prior to the machining of the at least one lens surface, fastening a lens holding block on the lens surface that is facing away from the lens surface that is to be machined, the lens holding block preferably only being removed again after the creation of the diffractive microstructure. Consequently, in a preferred embodiment of the invention the same block as in the possibly preceding PLP machining steps is in introducing the diffractive microstructure. This on the one hand allows the geometrical correlation between the two steps (and consequently the geometrical correlation between the local distributions of the specifications for the refractive and diffractive effects) to be ensured easily and with particularly high accuracy. On the other hand, there is no need for the process steps of deblocking and subsequent reblocking. When forming technologies are used, the block may in this case represent at least part of the primary-side stamp.

If the prescription surface (for sample back surface) is a surface that is not rotationally symmetrical, the modification of the height of the base area (sagitta) in dependence on the angle when applying the structure must be taken into account. This means in particular that when (CNC) turning is used, the tool is preferably adjusted correspondingly in the direction of the axis of rotation. When forming technologies are used, attention is preferably correspondingly paid to the orientation of the forming tool.

The invention is described below on the basis of preferred embodiments with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic representation of lenses, in particular spectacle lenses, with a diffractive microstructure on a prescription lens surface that have been produced by a method according to a preferred embodiment of the present invention;

FIG. 2A shows a schematic representation of individual method steps in a production method given by way of example, according to a preferred embodiment of the present invention; and

FIG. 2B shows a schematic representation of individual method steps in a production method given by way of example, according to a further preferred embodiment of the present invention.

FIG. 1 illustrates examples of spectacle lenses 10 of different effects and surface curvatures. The spectacle lenses 10 concerned respectively have a front surface 14 (also referred to here as the primary surface) and a back surface 16 (surface on the eye side, also referred to here as the secondary surface). The back surface 16 is in this case produced in particular as a prescription surface. In this case, the prescription surface 16 is formed with a diffractive microstructure 18, which in the material of the spectacle lens, that is to say in the body of the spectacle lens, that forms part of a spectacle lens blank, for example a molded spectacle lens blank, in production. The representation of the diffractive microstructure 18 in FIG. 1 is intended to be regarded as purely schematic. In particular, the respective microstructure 18 is not shown to scale. The microstructure 18 is usually much smaller in comparison with the body of the spectacle lens. Typical dimensions of diffractive microstructures are preferably about 0.3 to 5 μm in the axial direction (that is to say in the direction of the thickness of the spectacle lens) and about 1 to 500 μm in the lateral direction. Shown by way of example, one after the other from left to right, in FIG. 1 are a positive lens with a convex base curve (front surface 14), a negative lens with a convex base curve (front surface 14), a positive lens with a planar base curve (front surface 14) and a negative lens with a planar base curve (front surface 14).

In FIG. 2A, an example of a production method according to a preferred embodiment of the invention is illustrated. Thus, in this preferred embodiment a spectacle lens blank that already has the ultimately desired form of surface, for example on the front surface, is first provided in a step ST10, while the back surface still has to be modified in its surface form to create the desired prescription surface. In the preferred embodiment that is represented in FIG. 2A, this takes place in a step of its own (or multiple steps) ST12, in which the prescription surface is produced with the required refractive effect, for example by milling and/or turning and/or grinding. The prescription lens surface in this case represents a surface form that is specified in its required spherical and/or astigmatic effect and forms a modified surface of the lens (adapted to specified values) from the spectacle lens blank or semifinished product. In particular, this is a surface that is adapted to the individual prescription for a spectacle wearer. The specified values may in this case be provided in particular by local sagittae as a result of an individual surface calculation. In step ST12, consequently, varying amounts of material are removed, preferably locally, in order ultimately to achieve the required sagittae of the prescription surface. In the preferred embodiment of FIG. 2A, the diffractive microstructure is not yet obtained after step ST12. The resultant prescription surface however may still have such a great roughness that it would be inadequate for a final spectacle lens with conventional quality requirements.

In a subsequent step ST14, on the basis of this possibly still quite rough prescription surface a diffractive microstructure is then created on the prescription surface. In particular, this diffractive microstructure is created in the material of the spectacle lens (or spectacle lens blank). The introduction of the microstructure may in particular happen in the way already described further above by machining or forming. In this case it is possible to dispense with complex polishing of the prescription surface (in particular after step ST12).

In particular whenever a machining method or some other method involving sufficient modification of the surface of the blank toward the prescription surface (for example by removal of material or forming) is used for the creation of the microstructure 18, it is possible in a further preferred embodiment to dispense with any preceding machining of the spectacle lens blank to create the prescription surface in a method step of its own (in particular with tools of its own). This is schematically represented in particular in FIG. 2B.

Also in this embodiment according to FIG. 2B, a spectacle lens blank that already has the ultimately desired form of surface, for example on the front surface, is first provided in a step ST20, while the back surface still has to be modified in its surface form to create the desired prescription surface. In the preferred embodiment that is represented in FIG. 2B, this however takes place together with the creation of the diffractive microstructure in a single working step ST22. It is consequently also possible in this case to dispense with complex polishing, and even with grinding of the prescription surface.

In a preferred embodiment, a protective layer is deposited on the diffractive microstructure. If the microstructure is to become optically effective, this protective layer must differ in the refractive index and be sufficiently thick to become optically effective. Ideally, it is also used as a hard layer for the finished product.

Altogether, the invention presented here particularly concerns a method for providing spectacle lenses that have a microstructure on at least one side. A procedure in which the structure is introduced into a surface of the lens after the surface has been machined is described. This method offers the advantage in comparison with other methods that machining steps that serve to achieve the necessary optical quality of the corresponding surface can be omitted. Mentioned as a specific exemplary embodiment is the microstructuring of the back surface of a spectacle lens by CNC controlled turning after an initial formative machining, with the polishing step being omitted.

The invention presented here concerns a method for providing lenses, in particular spectacle lenses, that have a microstructure on at least one side. In this case, the invention particularly offers a method in which no special semifinished products are required, in which the structure does not have to be taken into account in the machining of the surface to be structured and in which at least the last polishing step can be omitted.

LIST OF DESIGNATIONS

-   10 spectacle lens -   14 front surface (primary surface) of the spectacle lens -   16 back surface (secondary surface) of the spectacle lens -   18 diffractive microstructure 

1. A method for producing a lens, comprising: providing a lens blank; and machining at least one lens surface of the lens blank to create a prescription lens surface of the lens, the machining of the at least one lens surface comprising creation of a diffractive microstructure.
 2. The method as claimed in claim 1, wherein the lens is a spectacle lens and the prescription lens surface is the back surface of the spectacle lens.
 3. The method as claimed in claim 1, wherein the machining of the at least one lens surface for the creation of the prescription surface comprises removal of material with a thickness of at least partially at least about 20 μm.
 4. The method as claimed in claim 1, wherein the machining of the at least one lens surface for the creation of a prescription lens surface comprises at least one of milling, turning, and grinding, and the creation of the microstructure takes place after the at least one of milling, turning, and grinding, without any interim polishing of the at least one lens surface.
 5. The method as claimed in claim 4, wherein the roughness of the prescription surface before the creation of the microstructure has at least partially an R_(z) value of one of at least about 5 μm, at least about 10 μm, and at least about 20 μm, and/or an R_(a) value of one of at least about 0.5 μm, at least about 1 μm, at least about 2 μm, and at least about 5 μm.
 6. The method as claimed in claim 1, wherein the creation of the diffractive microstructure comprises machining with material removal that is at least partially greater than the structure size of the diffractive microstructure in the direction of removal.
 7. The method as claimed in claim 6, wherein the creation of the microstructure comprises removal of material with a thickness of at least partially at least about 20 μm.
 8. The method as claimed in claim 1, wherein the provision of the lens blank comprises molding.
 9. The method as claimed in claim 1, wherein the method comprises, prior to the machining of the at least one lens surface, fastening a lens holding block on the lens surface that is facing away from the lens surface that is to be machined, the lens holding block only being removed again after the creation of the diffractive microstructure.
 10. The method as claimed in claim 1, wherein the diffractive microstructure is formed in the material of the lens blank.
 11. The method as claimed in claim 1, wherein the machining of the at least one lens surface for the creation of the prescription surface comprises removal of material with a thickness of at least partially at least about 20 μm.
 12. The method as claimed in claim 1, wherein the machining of the at least one lens surface for the creation of the prescription surface comprises removal of material with a thickness of at least partially at least at least about 0.1 mm.
 13. The method as claimed in claim 1, wherein the machining of the at least one lens surface for the creation of the prescription surface comprises removal of material with a thickness of at least partially at least about 0.5 mm.
 14. The method as claimed in claim 1, wherein the machining of the at least one lens surface for the creation of the prescription surface comprises removal of material with a thickness of at least partially at least about 2 mm.
 15. The method as claimed in claim 6, wherein the creation of the microstructure comprises removal of material with a thickness of at least about 0.1 mm.
 16. The method as claimed in claim 6, wherein the creation of the microstructure comprises removal of material with a thickness of at least about 0.5 mm.
 17. The method as claimed in claim 6, wherein the creation of the microstructure comprises removal of material with a thickness of at least about 2 mm. 